linux-kernel - Re: [patch] i386, numaq: enable TSCs again

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Message-ID: <20070525082259.GH8094@one.firstfloor.org>
Date:	Fri, 25 May 2007 10:22:59 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	William Lee Irwin III <wli@...omorphy.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Satyam Sharma <satyam.sharma@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch] i386, numaq: enable TSCs again

On Fri, May 25, 2007 at 01:19:44AM -0700, William Lee Irwin III wrote:
> On Fri, May 25, 2007 at 10:08:27AM +0200, Ingo Molnar wrote:
> > Andi, Andrew, do you remember why we disabled TSCs on NUMAQ? It was 
> > slightly async between CPUs, right? In that case we should try the patch 
> > below.
> 
> I remember. It was far beyond "slightly async;" they would drift
> minutes apart during reasonable amounts of uptime, though it would take
> at least several days to drift so far (I don't recall how long it took).

sched_clock should handle that.

> TSC synchronization is uniformly impossible on NUMA-Q. Bootlogs showing
> the results of the attempts are still extant. They shouldn't end up too
> far apart right after booting, but I don't have even ballpark estimates.
> I'd hazard a guess of a few seconds.

You should mark_tsc_instable(), but not tsc disable. In a release or two
hopefully even that will be obsolete.

-Andi
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